This invention relates to improvements in buoyant structures of the type particularly useful in undersea environments. More particularly, it relates to a novel skin for known buoyancy materials which provides an economical and low-density means of imparting strength and impact resistance to the buoyancy materials.
U.S. Pat. No. 3,622,437 to Sidney D. Cook, the disclosure of which is incorporated herein by reference, discloses a buoyancy material which exemplifies the type of structure with which the present invention is concerned. The structure disclosed therein comprises a plurality of generally spherical, low density, buoyant bodies encased in a matrix of a light weight material known as syntactic foam, i.e., a hardenable resin loaded with hollow microspheres which serve to lower its density.
Such buoyant materials have found many uses as the sea has increasingly been utilized as a resource. For example, they are used to impart buoyancy to submergible equipment such as deep sea research instruments, cables, pipelines, and marine riser pipe such as are used in off-shore oil drilling operations. Their commercial success may be traced primarily to the fact that they provide a relatively inexpensive buoyancy material which is capable of withstanding relatively high hydrostatic pressures. Furthermore, unlike single-walled hollow pressure vessels which are subject to catastrophic failure when the wall is penetrated, the buoyancy materials with which the instant invention is concerned comprise a compartmentalized structure which tends to localize any failure due to implosion and thereby to retain most of its buoyancy.
Such materials are manufactured by packing hollow or foam filled, thin walled, generally spherical buoyant bodies ranging, in general, from 1/16 inch to about 6 inches in diameter, in a mold of a desired shape, and thereafter filling the interstices among the buoyant bodies with a syntactic foam. The syntactic foam serves as a matrix to encase the implodable buoyant bodies and reinforces the walls thereof. However, as will be apparent from the foregoing description of their method of manufacture, buoyant bodies located near surface areas of such materials remain unreinforced since only a thin syntactic foam layer or no syntactic foam layer is interposed between the walls of the buoyant bodies and the surface of the finally formed article. In this circumstance, the hydrostatic pressure to which the materials can be subjected is significantly diminished, since the buoyant bodies nearest the surface remain subject to implosion.
Additionally, such materials are vulnerable to impact damage during transit and handling prior to use which may produce surface areas particularly vulnerable to implosion. In this regard, it should be noted that such materials are frequently fashioned to form massive flotation devices which can weigh upwards of a ton. Obviously, such devices are subjected to extraordinarily harsh conditions. During transit and installation, there are numerous dangers of impact damage, and thereafter such devices are subjected to harsh, undersea high pressure environments.
It is well known that strength and impact resistance of buoyant materials of the type described may be improved by applying a protective skin, e.g., a fiberglass resin laminate. However, this type of skin, which must be excessively thick in order to provide suitable protection to the buoyant bodies, is characterized by a substantially increased weight.
In most prior art structures, the fiberglass laminate skin is applied to the otherwise completed buoyancy material. This necessitates suitable surface preparation, e.g. sand blasting or abrasive treatment, prior to application and significantly increases the cost of the final product. Alternatively, the fiberglass can be applied to the interior of the mold and impregnated during molding in a manner similar to the process used in forming the unique skin which is the subject of this invention. However, in this case, the thickness of the fiberglass laminate required is excessive if a performance equivalent to that of the present invention is desired. This excessive thickness is costly in terms of fiberglass and labor required. However, the excessive weight is its major disadvantage.
Additionally, it has been observed that an externally applied fiberglass laminate may not provide the kind of protection required. This is because, at high hydrostatic pressures, water inevitably penetrates the laminate and reaches the interface between the laminate and the buoyant core material. When the device is returned to the relatively low pressure surface environment, the depressurization induces delamination.
From the foregoing, it will be apparent that an ideal buoyant structure should be capable of withstanding the hydrostatic pressure characteristic of the ocean depth in which it is to be used without implosion, should be strong and resistant to damage caused by impact encountered during transit and handling, should not absorb significant quantities of water in use, should have a low density (thus a high buoyancy), and should be inexpensive to manufacture.